Existing dry cleaning processes function by mechanically agitating articles to be cleaned, e.g., clothes, and a solvent. Typically, articles of clothing are placed in a container or basket with an amount of a chemical solvent that loosens dirt and dissolves staining matter from the clothes. The clothes are then agitated by movement of the basket to increase the effectiveness of the cleaning process. The agitation is often in the form of rotation, and rotation with an axis in the horizontal plane makes use of gravitational forces to further increase the amount of agitation.
Many chemical solvents are environmentally hazardous and present public health and safety risks. As a result, a number of solvents have been banned by law or heavily regulated. In addition, "environmentally friendly" alternatives have been sought. One such alternative is using liquid carbon dioxide (CO.sub.2) as a solvent.
Dry cleaning systems and processes using liquid/supercritical dense-phase gas such as carbon dioxide (CO.sub.2) are known in the art. In such processes, liquid CO.sub.2 is pumped throughout the system using a heavy-duty positive displacement pump. Specifically, liquid CO.sub.2 is pumped from a reservoir into a cleaning chamber where articles come into contact with the CO.sub.2. The articles are cleaned by agitation, such as by rotation of a container holding the articles, and finally, the liquid CO.sub.2 is pumped back into the reservoir. The pump is also used during additional steps of the dry cleaning process as are known in the art.
The use of such a pump has a number of disadvantages that render prior art systems complex and/or cost-inefficient for many applications. One disadvantage is that the pump is a relatively expensive element of the dry cleaning system. Another disadvantage is that the pump requires a net positive suction head ("NPSH"). This head is generated by both the fluid level in whatever vessel is to be drained and the elevation of the vessel relative to the pump inlet. Configurations that provide adequate pressure such as tall vessels or mounting the vessel about the pump are not desirable because they result in a large machine. Furthermore, completely draining the cleaning chamber still may be difficult because NPSH decreases as the chamber empties.
Another method of providing adequate pump head is by using a distillation chamber. Gas is heated in the chamber, and the resultant pressure increase is used to provide the desired NPSH. However, the use of such a distillation chamber adds complexity and cost to the system.
Furthermore, the pump is susceptible to damage and wear from dirt suspended in the fluid, which reduces the pumping efficiency. Filters cannot be used on the suction side of the pump because they decrease the pressure at the pump inlet, adding to the problem of attaining adequate positive pressure head. Thus, in addition to equipment and operating costs, frequent maintenance is also necessary.